At Russia's notorious Black Dolphin Prison on the border of Kazakhstan, inmates share small 50-square-foot cells that are set back behind three sets of steel doors. Inmates live in a "cell within a cell”, with 24-hour surveillance. Black Dolphin houses the most brutal criminals, including serial killers, cannibals, and terrorists. A prison lieutenant told National Geographic, which did a documentary on the facility, that the only way to escape is by dying. If you combine all the crimes of the inmates, they have killed about 3,500 people. That's an average of five murders per inmate.